Use 19 rexes and a yuty. Rexes should have ascendant saddles ideally. Attack it fast and quick. Surround it as soon as you can. When you get in. Make your rexes follow you and run around til it lands. Soon as it lands, whistle attack on it and use your yuty attack buff. You should be able to do it in 1 go. If on the off chance it flies off. Keep your rexes with you. Cause if they fall in the lava, they can't get out.

Be warned. Check every Dino when you've killed it and check the dragon. Recently the element has been vanishing so make sure you find it before the teleport takes you out of the arena